drm/vc4: hdmi: Fix no video output on DVI monitors

The drm edid parser doesn't signal RGB support on DVI monitors
with old edid versions, leading to 8-bit RGB mode being rejected
and no video on DVI monitors.

As 8-bit RGB is mandatory on HDMI and DVI monitors anyways we can
simply drop the RGB format check, aligning vc4 with other drivers.

Signed-off-by: Matthias Reichl <hias@horus.com>